Ion source having critically dimensioned extraction means



Nov. 28, 1967 R. LE BIHAN ET AL 3,355,615

ION SOURCE HAVING CRITICALLY DIMENSIONED EXTRACTION MEANS Filed April 15, 196s 5 'SheetsfSheet 1 INVENTORS R. LE Ell-MN 6 DMJUG/S Nov. 28, 1967 BlHAN ET AL 3,355,615

ION SOURCE HAVING CRITICALLY DIMENSIONED EXTRACTION MEANS Filed April 15, 1965 3 Sheets-Sheet 2 FIG 3.

INVENTORSI R. LE BIHA/V & amuals v ATTORNEY NOV. 28, 1967 LE BlHAN ET AL 3,355,615

ION SOURCE HAVING cRITIcALLY DIMENSIONED EXTRACTION MEANS Filed April m 1965 3 Sheets-Sheet s INVENTORS R. LE B/HA/V 6 D-MAUG/S United States Patent Ofi ice 3,355,615 Patented Nov. 28, 1967 4 Claims. 61. 31363) The present invention relates to ion source, and especially to an improved mode of extracting the ions from the source.

It is an object of this invention to provide an ion source capable of producing an intense beam of ions.

Another object of this invention is to provide an ion source capable of producing a high degree of ionization.

Still another object of this invention is to provide an ion source of high efiiciency.

A further object of the present invention is to provide an ion source in which the power required for heating the source is relatively reduced.

A still further object of the invention is to provide an ion source of relatively simple construction.

In accordance with the invention the above objects are achieved by providing an ion source including an enclosure, means for producing a plasma in said enclosure by ionization of a gaseous substance, and means for extracting the ions from said enclosure, characterized by the fact that the extracting means comprise one or several grids having extraction holes of circular or other shape, the value of the diameter or the width of the holes being, in meters, equal to or less than 2R where T being the temperature in degrees Kelvin of the enclosure, and 11 the number of ions or electrons per m. of the plasma, the thickness of the grids being of the same order of magnitude as the diameter or the width of the holes.

The invention will be more fully understood from the following description and the accompanying drawings.

FIG. 1 represents diagrammatically an example of an embodiment of the invention,

FIGURES 2 and 3 explain the principle of the invention, and

FIG. 4 is an example of an abacus usable in achieving the invention.

Referring to FIG. 1, there is represented an ion source comprising an ionization chamber or enclosure 1 having tantalum walls 2 that may be brought to a desired temperature for example, by means of a heater filament 3.

The enclosure 1 communicates 'by way of a channel 4 with a reservoir of caesium 5 which may likewise be brought to a desired temperature by appropriate means, not shown in this figure.

The upper Wall of enclosure 1 is partially apertured and the apertures are covered by grids 6.

An extractor electrode 7, disposed above enclosure 1 and set at a negative potential with respect to the enclosure by means of a source of potential (not shown) comprises apertures facing the grids 6.

In operation, caesium vapor, produced by heating the reservoir 5, penetrates into enclosure 1 which is maintained at a high temperature, say between 1400 K. and 2200 K., and the caesium atoms are dissociated into electrons and positive ions. This phenomenon, known as surface ionization or contact ionization, results in the formation of a plasma in the interior of enclosure 1.

Now the plasma, that comprises electrons, positive ions also a certain proportion of neutral atoms, is not in contact with the walls of enclosure 1 because a sheath, composed to a great extent of ions, is comprised between the walls and the plasma if the work function of the walls is higher than the plasma potential. The ion density (number of ions per unit volume) is therefore considerably higher in the sheath adjacent the walls than in the plasma which occupies the remainder of the enclosure.

FIGURES 2 and 3 permit to see what happens when the radius of the extraction orifice does or does not exceed the thickness of the ion sheath. In these two FIG- URES 2 and 3, the plasma is designated by reference numeral 8, the enclosure wall by 2, and the ion sheath, indicated by the signs is designated by the reference 9.

In FIGURE 2, the radius of the orifice provided in wall 2 is greater than the thickness of sheath 9. Within this orifice the ions of sheath 9 surround a portion of plasma 8. On the contrary, in the case of FIGURE 3 where the orifice radius is equal to the thickness of sheath 9, only the ions of the sheath traverse the orifice without any portion of the plasma being drawn along. As a result, the mean density of the ion current and the ionization degree are much higher in the case of FIG. 3 than in the case of FIG. 2. I

In accordance with the invention, the orifices of the grids 6 (FIG. 1) are given a radius equal to or less than the thickness R of the ion sheath.

The thickness R may be predetermined by the formula Ra g mentioned above, the value of n being given 'by the relation where n,; is the density of the metal vapor in the enclosure prior to the ionization, t=the temperature of the enclosure and V =the ionization potential of the metal vapor. The units employed are those of the international system (M.K.S.A.).

FIGURE 4 is an abacus indicating the thickness R, in microns, of the ion layer in a tantalum enclosure versus the temperature T (in degrees Kelvin) of a caesium reservoir for different temperatures T of the tantalum. It may be seen on this abacus that, for example, for tantalum walls heated at 1600 K. and a reservoir of caesium at 425 K. the thickness of the ion sheath is 2071.. Under these conditions use is made, in accordance with the invention, of extraction orifices of a diameter that does not exceed 40 i, and thus ion currents of the order of 40 ma./cm. are obtained.

In accordance with the invention, the thickness of grids 6 is given a value of preferably the same order as the diameter or the width of the orifices. It has been found, in effect, that the ion current is not affected by the thickness of the grids. On the other hand, it is obvious that thick grids (deep holes) bring about pressure losses which are practically inexistent in thin grids (non-deep holes), so that with thin grids it sufiices to heat the reservoir that supplies the metal vapor, to a lower temperature than in the case of thick grids.

In the example described, caesium and tantalum have been mentioned, but instead of caesium other metals may be used that are easy to ionize (potassium, lithium, rubidium, sodium, etc.) and the tantalum may be replaced by other metals having a high work function (tungsten, molybdenum, rhenium, etc.). Thus with a tungsten enclosure, heated at 1800 K., it is possible by applying the principle of the invention, to extract an ion density of the order of 500 ma./cm. and more than 1 a./cm. at 1900 K.

The extraction orifices may have a cross-section of any desired shape: circular, square, rectangular, triangular, etc., and the grids may be replaced by nets of parallel wires, separated by narrow intervals. In all cases, the ion extraction is effected, in accordance with the invention, through passages whose width does not exceed the double of the thickness of the ion sheath, adjacent the walls of the isothermal cavity.

While this invention has been described in a specific embodiment using surface ionization, it will be obvious to those skilled in the art that the invention applies generally to all types of ion sources comprising an enclosure or ionization chamber in the interior of which a plasma is formed. Thus the invention is not to be limited to the details shown, except as defined in the following claims.

We claim:

-1. An ion source including an enclosure, means for producing a plasma in said enclosure by ionization of a gaseous substance, andmeans for extracting the ions from said enclosure, said extracting means including at least one grid provided with substantially circular extraction holes having a diameter at most equal, in meters,

to 2R where T being the temperature in degrees Kelvin of the enclosure, and n the number of ions or electrons per m. of the plasma.

T being the temperature in degrees Kelvin of the enclosure, and n the number of ions or electrons per m9 of the plasma.

4. An ion source as claimed in claim 3, wherein the thickness of said grids is substantially equal to the width of said holes.

References Cited UNITED STATES PATENTS 5/1965 Hoyer et al. 313-61 8/1966 Sunderland et al. 31363 DAVID J. GALVIN, Primary Examiner.

JAMES W. LAWRENCE, Examiner.

S. A. SCHNEEBERGER, Assistant Examiner. 

3. AN ION SOURCE INCLUDING AN ENCLOSURE, MEANS FOR PRODUCING A PLASMA IN SAID ENCLOSURE BY IONIZATION OF A GASEOUS SUBSTANCE, AND MEANS FOR EXTRACTING THE IONS FROM SAID ENCLOSURE, SAID EXTRACTING MEANS INCLUDING AT LEAST ONE GRID PROVIDED WITH EXTRACTION HOLES HAVING A WIDTH AT MOST EQUAL, IN METERS, TO 2R WHERE 